Muon g-2 Theory Initiative

Muon g-2 Theory Initiative
Name	Muon g-2 Theory Initiative
Formation	2012
Type	Scientific collaboration
Headquarters	International
Leaders	""

Muon g-2 Theory Initiative The Muon g-2 Theory Initiative is a coordinated international effort to compute the Standard Model prediction for the anomalous magnetic moment of the muon, a_μ, with sufficient precision to confront high-precision measurements. It brings together theoretical physicists from institutions involved in particle physics, lattice gauge theory, and phenomenology to reduce uncertainties arising from hadronic vacuum polarization and hadronic light-by-light contributions.

Background and Motivation

The Initiative originated in response to persistent discrepancies between experimental determinations of a_μ from the Brookhaven National Laboratory experiment and theoretical predictions informed by results from CERN, Fermilab, and global accelerator facilities. Prominent drivers include analyses related to results produced by teams associated with the Muon g−2 experiment, historical measurements by the E821 experiment, and theoretical frameworks developed at institutions such as Princeton University, Massachusetts Institute of Technology, California Institute of Technology, Stanford University, University of Cambridge, University of Oxford, and Imperial College London. Motivations also trace to precision tests proposed in contexts like the Standard Model (particle physics), tensions reminiscent of anomalies discussed around the LEP program, and implications for beyond-Standard-Model proposals advanced at conferences such as the International Conference on High Energy Physics and workshops at CERN Theory Department.

Organization and Membership

Membership comprises researchers affiliated with universities and laboratories including University of Washington, University of Mainz, Brookhaven National Laboratory, Fermi National Accelerator Laboratory, RIKEN, University of Bern, ETH Zurich, University of California, Berkeley, Columbia University, Università di Roma La Sapienza, University of Liverpool, University of Southampton, University of Manchester, University of Edinburgh, Seoul National University, Tata Institute of Fundamental Research, University of Tokyo, CEA Saclay, Max Planck Institute for Physics, DESY, Kavli Institute for Theoretical Physics, Perimeter Institute for Theoretical Physics, SLAC National Accelerator Laboratory, Los Alamos National Laboratory, NIKHEF, Budker Institute of Nuclear Physics, Institute for Nuclear Research (Russia), National Taiwan University, Australian National University, University of Melbourne, Sungkyunkwan University, University of Pisa, Scuola Normale Superiore di Pisa, Université Paris-Saclay, Sorbonne University, École Polytechnique, University of Warsaw, Jagiellonian University and many other centers. Leadership and working groups have included specialists who have published in journals associated with Physical Review Letters, Physical Review D, Journal of High Energy Physics, Physics Letters B, and European Physical Journal C.

Scientific Goals and Objectives

The Initiative's principal objective is to provide a robust, consensus Standard Model prediction for a_μ by combining inputs from dispersion relations tied to cross sections measured at facilities like VEPP-2000, DAΦNE, and BESIII with first-principles calculations from lattice gauge theory groups affiliated with FLAG-related collaborations and lattice consortia at RBC/UKQCD, BMW collaboration, ETM Collaboration, and others. It aims to quantify uncertainties linked to hadronic vacuum polarization (HVP) and hadronic light-by-light (HLbL) scattering contributions, reconcile data-driven frameworks developed using results from CMD-3 and SND detectors, and provide covariance matrices useful for global fits in contexts explored at ICHEP and Moriond meetings.

Methodology and Calculations

Methodologies combine dispersion-theory techniques originating in analyses by researchers associated with Cornell University and University of Bonn with lattice quantum chromodynamics (QCD) computations using algorithms and resources from projects at Blue Gene facilities, JUQUEEN, Summit (supercomputer), and national supercomputing centers such as NERSC. Calculations use chiral perturbation theory inputs developed by groups connected to University of Bern and University of Mainz, finite-volume corrections studied by teams at University of Edinburgh and Swansea University, and renormalization techniques employed by researchers at DESY and IHEP (China). Cross-checks involve perturbative QED results from studies linked to Harvard University and multi-hadron contributions informed by analyses from Rutgers University and University of California, Santa Barbara.

Key Results and Impact

Key outputs include community white papers synthesizing consensus estimates for HVP and HLbL that influenced interpretations of the discrepancy between experiment and theory reported by Fermilab and earlier by Brookhaven National Laboratory. Results have catalyzed further theoretical research at institutions such as MIT, Yale University, University of Chicago, Princeton University, and stimulated model-building in beyond-Standard-Model venues including work by researchers associated with CERN Theory Group and DESY. The Initiative's assessments have been cited in review articles and have shaped strategies for precision tests discussed at symposia hosted by Perimeter Institute for Theoretical Physics, KITP, and Aspen Center for Physics.

Collaborations and Experimental Connections

The Initiative coordinates closely with experimental collaborations at Fermilab, Brookhaven National Laboratory, VEPP-2000 (Novosibirsk), DAΦNE (Frascati), and BEPCII via shared data and recommended procedures for combining e+e− → hadrons cross-section measurements from experiments like KLOE, BaBar, Belle, CMD-3, SND, and BESIII. It has interactions with lattice collaborations such as BMW collaboration, RBC/UKQCD, ETM Collaboration, JLQCD, and HPQCD to compare methodologies, and engages with funding and coordination bodies including European Research Council, Simons Foundation, US Department of Energy, Science and Technology Facilities Council, and national science agencies across Europe, Asia, and the Americas.

Controversies and Open Issues

Contested points remain over the size of the HVP contribution when derived from e+e− data versus lattice-QCD results, debates that have involved groups at BMW collaboration, RBC/UKQCD, HPQCD, and proponents of data-driven approaches tied to experiments like BaBar and KLOE. Disagreements prompted exchanges at forums including ICHEP, Moriond, and workshops at CERN and raised questions relevant to interpretation frameworks used by theorists at University of Mainz and University of Bern. Open issues include control of isospin-breaking effects studied at University of Bonn and University of Southampton, radiative corrections analyzed by experts at Università di Roma La Sapienza and University of Pisa, and systematic uncertainties in lattice computations explored by teams at DESY and ETH Zurich.

Category:Particle physics collaborations